The Twenty-second United States Census, known as Census 2000 and conducted by the Census Bureau, determined the resident population of the United States on April 1, 2000, to be 281,421,906, an increase of 13.2% over the 248,709,873 persons enumerated during the 1990 Census. This was the twenty-second federal's census and was at the time the largest civility administrative peacetime effort in the United States.

Approximately 16 percent of households received a "long form" of the 2000 census, which contained over 100 questions. Full documentation on the 2000 census, including census forms and a procedural history, is available from the Integrated Public Use Microdata Series.

Microdata from the 2010 census is freely available through the Integrated Public Use Microdata Series. Aggregate data for small areas, together with electronic boundary files, can be downloaded from the National Historical Geographic Information System.

The U.S. resident population includes the total number of people in the 50 states and the District of Columbia. The Bureau also enumerated the residents of the associated state of Puerto Rico; its population was 3,808,610, an 8.1% increase over the number from a decade earlier.

The United States Census Bureau (officially the Bureau of the Census, as defined in Title 13 U.S.C. § 11) is the government agency that is responsible for the United States Census. It also gathers other national demographic and economic data. As part of the United States Department of Commerce, the Census Bureau serves as a leading source of data about America's people and economy.

The most visible role of the Census Bureau is to perform the official decennial (every 10 years) count of people living in the U.S. The most important result is the reallocation of the number of seats each state is allowed in the House of Representatives, but the results also affect a range of government programs received by each state. The agency director is a political appointee selected by the President of the United States.

The Constitution of the United States (Article I, section II) directs that the population be enumerated at least once every ten years and the resulting counts used to set the number of members from each state in the House of Representatives and, by extension, in the Electoral College. The Census Bureau now conducts a full population count every 10 years in years ending with a 0 (zero) and uses the term "decennial" to describe the operation. Between censuses, the Census Bureau makes population estimates and projections.